Actually, Serious Sam and particularly System Shock are different enough from Doom that their comparison isn't as strong as Road Rage to Crazy Taxi. If you compare Hit & Run to Grand Theft Auto 3, you will see that they share a lot of things in common as well-- but I don't think enough to warrant a lawsuit. In the games industry to win a lawsuit like this you would have to prove that the company literally stole your game design and barely changed it (not redoing it completely into a Simpsons motif, and creating new abilities, things to do, etc that the original game lacked). Otherwise we would never have seen Mortal Kombat after Street Fighter 2 came out...